1. Technical Field
The present invention relates to a seat belt retractor for use by an occupant of a vehicle, and particularly to a seat belt retractor having a spool which is driven by an electric motor.
2. Background Art
A known seat belt retractor having a spool driven by an electric motor is disclosed in U.S. Pat. No. 4,659,108. The retractor includes a spool and seat belt webbing wound around the spool. A tongue is connected to the webbing and is insertable into a buckle attached to the vehicle after the webbing is drawn around an occupant. The occupant must overcome the inertia of the motor while manually withdrawing the webbing from the retractor to insert the tongue into the buckle. The motor is actuated to rotate the spool in the belt retraction direction in response to the tongue being inserted into the buckle. Thus, the webbing begins to tighten against the occupant. As the webbing tightens against the occupant, the tension in the webbing increases. When the tension increases to a sufficient amount, the motor is deactuated. After a predetermined time delay, the motor is actuated to rotate the spool in the belt withdrawal direction for a sufficient time period to pay out a small amount of webbing. The payout of the webbing relieves the force applied by the webbing against the occupant. In the event of a crash condition, a conventional inertia actuated locking pawl engages the teeth of ratchet wheels connected to the spool so as to lock the spool against rotation in the belt withdrawal direction.
Another known seat belt retractor having a spool driven by an electric motor is disclosed in U.S. Pat. No. 4,655,312. The retractor includes a spool and seat belt webbing wound around the spool. A tongue is connected to the webbing and is latchable to a buckle attached to the vehicle after the webbing is drawn around an occupant. The occupant must overcome the inertia of the motor while manually withdrawing the webbing from the retractor to latch the tongue to the buckle. The motor is actuated to rotate the spool in a belt retraction direction in response to the tongue being latched to the buckle. Thus, the webbing begins to tighten against the occupant. While the webbing tightens against the occupant, the tension in the webbing increases. When the tension in the webbing increases above a predetermined amount, the motor is actuated to rotate the spool in a belt withdrawal direction. The spool rotates for a time period sufficient to pay out a relatively small predetermined amount of webbing. Slack is thereby established in the webbing. The motor is deactuated after the slack in the webbing is established. In the event of a crash condition, a conventional inertia actuated locking pawl engages the teeth of ratchet wheels attached to the spool. The spool of the retractor is thereby locked to prevent movement of the webbing in the belt withdrawal direction.